tweak tree roam comment.

src/chart/tree/TreeSeries.js

@@ -133,7 +133,7 @@ export default SeriesModel.extend({
         // the layout of the tree, two value can be selected, 'orthogonal' or 'radial'
         layout: 'orthogonal',
 
-        roam: false,
+        roam: false, // true | false | 'move' | 'scale', see module:component/helper/RoamController.
         // Symbol size scale ratio in roam
         nodeScaleRatio: 0.4,
 

test/tree-roam.html

@@ -69,6 +69,8 @@ under the License.
                                     bottom: '14%',
 
                                     roam: true,
+                                    // roam: 'scale',
+                                    // roam: 'move',
 
                                     layout: 'radial',